1. Field of the Invention
The present invention relates to humidification arrangements used to control the humidity of breathable gases used in all forms of respiratory apparatus ventilation systems including invasive and non-invasive ventilation, Continuous Positive Airway Pressure (CPAP), Bilevel therapy and treatment for sleep disordered breathing (SDB) conditions such as Obstructive Sleep Apnea (OSA), and for various other respiratory disorders and diseases.
2. Description of the Art
Respiratory apparatus commonly have means to alter the humidity of the breathable gas in order to reduce drying of the patient's airway and consequent patient discomfort and associated complications. The use of a humidifier placed between the positive airway pressure (PAP) device (or flow generator) and the patient mask, produces humidified gas that minimizes drying of the nasal mucosa and increases patient airway comfort.
Many humidifiers types have been proposed, including humidifiers that are either integrated with or configured to be coupled to the relevant respiratory apparatus. While passive humidifiers can provide some relief, generally a heated humidifier is required to provide sufficient humidity and temperature to the air so that patient will be comfortable.
Humidifiers typically comprise a water tub having a capacity of several hundred milliliters, a heating element for heating the water in the tub, a control to enable the level of humidification to be varied, a gas inlet to receive gas from the PAP device, and a gas outlet adapted to be connected to a gas conduit that delivers the humidified pressurized gas to the patient's mask.
However such a humidifier arrangement has a limited area of interaction between the surface of the water in the tub and the gas flowing above so that the capacity to deliver a high absolute humidity rapidly is limited. In addition the humidified air may cool on its path along the conduit from the humidifier to the patient, leading to the phenomenon of “rain-out”, or condensation, forming on the inside of the conduit.
Tub-of-water humidifiers are also vulnerable to liquid water spillage if they are not maintained in the vertical. Spillage of liquid water can either travel into the gas conduit to the patient or back into the PAP device and associated electronics or deplete the reservoir of humidifying water. In either of the cases the spillage of water is undesirable.
Several references describe humidification arrangements associated with the gas conduit between the blower or PAP device and the patient mask. These arrangements have consisted of discrete in-line units, tubes of water within the conduit or re-vaporization of water condensate in the gas conduit.
U.S. Pat. No. 4,146,597, U.S. Pat. No. 4,861,523, U.S. Pat. No. 5,367,604 and U.S. Pat. No. 6,010,118 disclose the use of a discrete in-line unit placed somewhere along the gas conduit to the patient. However such discrete humidification units have the disadvantages of being bulky to the patient and impeding the free movement of the gas conduit as the patient moves.
Another humidification scheme is to have a water source enclosed within a tube of a semi-permeable membrane which is then inserted into the gas conduit. The tube may also incorporate heating elements to aid in the generation of water vapour. Examples of such prior art are U.S. Pat. No. 3,871,373, U.S. Pat. No. 4,708,831 and U.S. Pat. No. 6,201,223.
Another example is the use of many tubes that are inserted into the gas conduit, U.S. Pat. No. 4,146,597, U.S. Pat. No. 6,394,084, U.S. Pat. No. 6,557,551 and U.S. Pat. No. 6,877,510. The disadvantage of such arrangements is the impediment to gas flow by the inserted tubes within the gas conduit to the patient. The tubes may also contribute to the cleaning burden within the gas conduit.
U.S. Pat. No. 6,662,802 describes an arrangement for collection of water condensation within the gas conduit and then its re-vaporisation into the gas flow, to reduce the effect of condensation within the gas conduit and to restore the humidity in the gas conduit. However, the apparatus does not have the capacity to increase the absolute humidity beyond what can be gathered as a water source from the condensate in the gas conduit. That is, the arrangement cannot increase the absolute humidity, merely restore it to the absolute humidity at which the gas is supplied to the conduit.
None of these prior art devices provides an entirely satisfactory solution to the provision of humidified breathable gas to the patient, nor to ease of construction and hygiene requirements for a humidification apparatus.